1. Field of the Invention
The present invention concerns a filter for an x-ray device to shape an intensity profile of x-ray radiation emanating from an x-ray source of the x-ray device. The invention also concerns an x-ray device that has at least one such filter.
2. Description of the Prior Art
In the acquisition of x-ray projections of a body region of a patient (in particular for diagnostic purposes), it is common practice to use shaping filters. Such a shaping filter is typically associated with an x-ray source in order to at least partially shade, attenuate or at least partially absorb the x-ray radiation emanating from the x-ray source. Such a shaping filter is normally designed such that specific peripheral body regions of a patient (such as the arm) that, due to their smaller thickness, have a lower radiation attenuation for x-ray radiation, are at least partially shaded given a central projection, for example. This means that the x-rays emanating from the x-ray source first penetrate the shaping filter and only then penetrate the shaded body region of the patient with lower intensity. Conversely, central regions are not shaded by the shaping filter.
Such an embodiment of a shaping filter has the advantage that specific body regions of the patient (such as the noted arms) are not exposed to an unnecessarily high intensity of x-ray radiation for the imaging. If the complete scan of a patient is considered, the dose of x-ray radiation applied to the patient can also be reduced in this way. Furthermore, the shaping filter produces a certain homogenization of the x-ray radiation passing through the patient and received by an x-ray detector, meaning that the shaping filter at least partially compensates the inherently inhomogeneous signal curve so that the noise in the x-ray detector also varies less from location to location, which is desirable.
Such, a shaping filter is normally designed for a defined, average patient geometry or a beam attenuation profile of an average patient, such that the effects of the dose savings and the homogenization lead to results of different quality in different patients. An additional disadvantage occurs in the case of static shaping filters in x-ray devices such as computed tomography apparatuses in which the shaping filter rotates with the x-ray source around a patient. Depending on the rotation angle or projection angle for the acquisition of x-ray projections, the patient geometry and the beam attenuation profile of the patient thereby will be different; so a static shaping filter cannot be adapted to such a dynamic situation.
In U.S. Pat. No. 7,088,799 B2 a filter for an x-ray device is described that is designed with a cylindrical shape, and has multiple filter profiles and is rotatable around the longitudinal axis of the cylinder.
From U.S. Pat. No. 7,330,535 B2 a filter arrangement for an x-ray device is known that has a pair of bow tie filters. The filters are executed in a u-shape and are pivotable around an axis.
In U.S. Pat. No. 7,688,936 B2 a filter arrangement is described that has a set of filters that are arranged on a hub that is rotatable around an axis. The individual filters in the set of filters differ from one another and are used with different tube voltages applied to an x-ray tube of a CT apparatus for a multiple-energy data acquisition.